In studies on multiple sclerosis, abnormal suppressor T-cells were detected to measles, rubella and occasionally to Epstein-Barr virus but not to cytomegalovirus or Herpes Simplex I virus. The response is more apparent during exacerbations but the response to measles remains aberrant even in some stable patients. The loss of suppressor T-cells to brain cell antigens, axolemma, and myelin occurs frequently with patients in exacerbation and may contribute to a possible autoimmune mechanism. Attempts to isolate an infectious agent from bone-marrow samples of MS patients proved negative. A few MS patients have CNS fluid interferon (IF) which can be detected. MS patients appear to have normal immunocompetence as measured by lymphocyte stimulation or interferon production in response to general mitogens and recall antigens in vitro during exacerbations, as well as in stable phase. Immune complexes have been detected in CSF of MS patients. The T gamma lymphocytes were found to participate in as well as respond to IF as natural killer (NK) cells. In tumor cell lines from JC infected owl monkeys, the JC large "T" antigen rapidly disappears from the cells upon serial cultures.